1. Field of the Invention
This invention pertains to a visibility meter such as is used to quantify visibility in the atmosphere for, e.g., aeronautical purposes. More particularly, this invention pertains to components which can be utilized in such a visibility meter, but which may have alternative applications in other fields, in which applications the characteristics of the components can be used to advantage.
2. Description of the Prior Art
Conventional visibility meters are constructed either as forward-scatter meters, or FSM's, or as backward-scatter meters, or BSM's. Regardless of the construction of such visibility meters, it is known that the light source should advantageously have a high intensity and a high local stability of the position from which light is emitted.
Such local stability is of particularly great importance in forward-scatter meters, since such meters are so constructed that the light receiver is prevented from receiving any light from the aureole or corona of the light source. In the event that light from the aureole or corona of the light source is allowed to be incident directly upon the receiver, the output of the receiver will not be dependent upon scattering of light in the atmosphere and will therefore produce undependable measurements of visibility. Therefore, it is especially important in the case of forward-scatter meters that light produced by the light source be confined as closely as possible to a single predetermined position and have high local stability there, in order to prevent changes in location of the aureole or corona from adversely affecting accuracy.
In the prior art, a spark-gap has been used as a light source. Although such a spark-gap light source has a desirably high intensity, its local stability is low because the position of the spark may change during repeated flashings of the light source.
It has also been known to utilize a gas discharge flash tube as a light source in visibility meters. Although such gas discharge flash tubes have high local stabilities, they do not produce light of sufficient intensity.
Furthermore, it is known that light in the infrared portion of the spectrum is scattered in the atmosphere to a lesser degree than light in the visible portion of the spectrum. Hence, to the extent that the receiver in a visibility meter is permitted to respond to light in the infrared portion of the frequency spectrum, accuracy of the visibility measurement will be degraded.
Therefore, it would be advantageous to provide components for use in a visibility meter which would be so constructed that (a) the light source would have both a high intensity and a high local stability, and (b) the visibility meter would be unresponsive to light in the infrared portion of the frequency spectrum.